<k-upload>-onSuccess: 2nd argument should contain api response but does not

Having my tiny custom field:

      <k-image :src="value" />
      <k-upload ref="upload" @success="onSuccess" />
      <k-button icon="upload" @click="upload">Upload</k-button>

and some methods with it:

  methods: {
    upload() {
        url: window.panel.api + '/media/freePicturesPost',
        accept: "image/*",
        multiple: true
    onSuccess (uploads, response) {
      // i.e.
      // etc.

Uploading my file works fine; here is the response I get from my own API endpoint:


What I am wondering is that in onSuccess() the second argument is undefined, but should contain the API response according to

The first parameter is not empty, but does not contain the api response.

Why that?